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. 2022 Aug 18;22(16):6194.
doi: 10.3390/s22166194.

Humidity Sensitivity of Chemically Synthesized ZnAl2O4/Al

Takayuki Nakane  1 , Takashi Naka  1 , Minako Nakayama  1 , Tetsuo Uchikoshi  1
Affiliations

Humidity Sensitivity of Chemically Synthesized ZnAl2O4/Al

Takayuki Nakane et al. Sensors (Basel). .

Abstract

Humidity sensitivity is evaluated for chemically synthesized ZnAl2O4/Al devices. We succeeded in synthesizing the ZnAl2O4/Al device by applying chemical techniques only. Hydrothermal treatment for the anodized aluminum (AlOx/Al) gives us the device of the ZnAl2O4/Al structure. All fabrication processes were conducted under 400 °C. The key was focusing on ZnAl2O4 as the sensing material instead of MgAl2O4, which is generally investigated as the humidity sensor. The evaluation of this ZnAl2O4/Al device clarified its effectiveness as a sensor. Both electrical capacitance, Cp, and the resistivity, Rp, measured by an LCR meter, obviously responded to the humidity with good sensitivity and appreciable repeatability. Our synthesis technique is possible in principle to improve on the process for the device with a complex structure providing a large surface area. These characteristics are believed to expand the application study of spinel aluminate devices as the sensor.

Keywords: ZnAl2O4; anodization; device design; environmental monitoring; gas sensor; humidity; hydrothermal synthesis; oxide; semiconductor.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
The flow of the sample preparation process with schematic illustrations of the cross-sectional image of the sample at each step and real pictures of them. The upper picture shows the scale (left side), electropolished Al plate (second from the left), anodized electropolished plate (AlOx/Al: second from the right) and hydrothermal synthesized sample (ZnAl2O4/Al: right side). Bottom picture shows the device after measurement.
Figure 2
Figure 2
Schematic illustration of the measurement configuration for evaluating the humidity sensitivity of the sample device.
Figure 3
Figure 3
XRD patterns of the ZnAl2O4/Al sample. The peaks of ZnAl2O4 were all indexed in this figure. On the other hand, the peak positions of Al were only indicated by a red dash line. The 400 and 440 peaks were overlapped with the small Al peaks.
Figure 4
Figure 4
SEM images of the surface of the anodized AlOx layer (left side) and hydrothermally synthesized ZnAl2O4 layer (right side).
Figure 5
Figure 5
Mapping images observed by SEM-EDS for ZnAl2O4/Al sample. Area 1 shown on the top was 3000 times scale, and Area 2 (bottom) was 13,000 times, respectively.
Figure 6
Figure 6
(A) Dependence of the Sc on the %RH in the atmosphere of ZnAl2O4/Al device. (B) Dependence of relationship between the Sc and %RH values on the applied AC frequency to ZnAl2O4/Al device. The inserted figure plots the Sc values at %RH = 52 against the applied AC frequency.
Figure 7
Figure 7
Dependence of the SR on the %RH in the atmosphere of the ZnAl2O4/Al device. The inserted figure plots the SR values at %RH = 52 against the applied AC frequency.
Figure 8
Figure 8
Repeatability of the Cp of ZnAl2O4/Al device against changing pressure between the vacuumed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Cp is plotted against absolute time. (B) Cp is plotted against the relative time of each cycle.
Figure 9
Figure 9
Repeatability of the Rp of ZnAl2O4/Al device against changing pressure between the vacuumed state as %RH = 0 and normal atmosphere as %RH = 59. (A) Rp is plotted against absolute time. (B) Rp is plotted against the relative time of each cycle.
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